Photoelectric assorting device



Sept. 27, 1938.

o. c. cox

PHOTOELECTRIC ASSORTING DEVICE Filed Nov. 14, 1935 H s 2 i N s Q a r F. W A 2 3 ON m on 3 S @w i 1' lEi a mm 4 N. 9 3 a a a i 3 a Q m Q 3 3 m on I I h LM mvmoa Dav/'0 C Cox ATTORNEY 5 Patented Sept. 27, 1938 PAT N OFFICE rno'roamcrmc assoamc DEVICE paid 0. Cox, Lowell, Mich, minor to Electric Mich;

Company, Grand Rapids,

' ali u auon November 14 1935', serial-manner 21 cum-n.1- chaos-111') The main objects oi this invention are: I

First, to provide a photoelectric assorting apparatus which is especially well adapted for assorting small articles such as beans.

Second, to provide means in a system of the foregoing character for automatically. maintaining extremely constant functioning even in the presence of gradual variations .of power supply voltages, photo-tube sensitivity, ambient temperature, lamp brightness and certain other factors, which variations would otherwise be intol erable or materially reflect in accuracy of the' apparatus.

Third, to provide an assorting machine with which great speed is attainable without consequent errors.

Fourth, to provide an assorting machine disposed to appraise any given one of the objects being assorted without error dueto the nature of a preceding object.

Fifth, to provide an assorting machine requiring a minimum of attention due, among other factors, to the use of a control dial bearing significant calibration.

Objects relating to details and economies of my invention will appear from the description to follow. The invention is defined and pointed out in the claims. I

Preferred embodiments of my invention are illustrated in the accompanying drawing, in

which:

Fig. 1 is a diagrammatic view of a photoelectric assorting device and circuit therefor embodying features of my invention.

Fig. 2 is a fragmentarytop plan view of the conveyor and associated parts.

Fig. 3 is similar to Fig. 1 and illustrates a modifled form of my invention. 1

Considerable skill and time are required to set an assorting machine for the most desirable separation. If, then, such a machine will not closely maintain this careful setting,-the amount of attention required becomes excessive. If direct current amplifiers are used in the ejecting circuit, very minute variations in lamp brightness, phototube sensitivity, Photo-tube resistor value, voltage supplies, etc., are magnified and largely displace background which usually appears between ob- -iects. The grid resistor which bleeds off current to avoidpermanent,displacement always bleeds oif a partof the working impulse, amplifying with poor fidelity the impulses received from the photo-tube. It canalso be shown that such circuits always involve errors due to the fact that after viewing a dark object they require a larger impulse to trip than after viewing a light object. Hereinafter, this is referred to as historical error, that is, error due to an improper appraisal of an object resulting from the nature of a preceding object. I

'The permanent displacement of machine set ting (as caused by a change in lamp brightness) in direct current amplifiers is exceedingly un- The present invention includes a circuit so designed as to automatically maintain the impulse required to trip the ejector essentially proportional to the intensity of illumination. Inasmuch as change in photo-tube sensitivity and change in photo-tube resistor value have the same effect as change in lamp brightness, they are likewise accurately compensated.

In a capacity-coupled amplifier, the grid resistor should be of very high value for faithful amplification of the working impulses. This same resistor should be of very low value while viewing the background, in order to assure the attainment of equilibrium before the succeeding object appears. -This disparity results ordinarily in a com promise value, neither high enough to accomplish the first object nor low enough to accomplish the second. Fair results may be attained by wide separation of objects being assorted so that the background is viewed much longer than the object is viewed. This imposes a serious capacity limitation.

In the circuit which is the preferred embodiment of the present invention, every one of the aforementioned discrepancies is reduced theoretically to zero, and with readily attainable manufacturing accuracy they are too small to be detectable in operation. The faithfulness of ampliflcation is as high as that of a direct current amplifier, the resetting following a change in lamp brilliance is closely proportional to said change, the background is successfully used as a comthronoh narison. standard, and in addition, the objects be ing assorted may follow each. other very closely and very rapidly. This circuit furthermore makes possible the calibration of the control dial in such figures as the percentage decrease in field brigh required to operate the ejector which immenseiy simplifies the maintenance of a large of machines.

the illustrated embodiment of my invention, i is a constantly moving continuous conveyor, in this case a drum, having a series of uniformly spaced article seats 2 each adapted to convey an article 3 to be inspected through an inspecting Zone t. The conveyor is in the form of a hollow drum joinnaled to rotate in the standards 5, one off the standards being hollow and connected to suction means (not shown) for creating a suction the article seats 2 for holding the articles thereon, the articles being picked up by the conveyor as the latter sweeps through an article supply means The article inspecting zone t comprises a houscontaining lamps ii for illuminating the articles and rim of the conveyor as the latter passes the zone, the light reflected therefrom directed to a photo-tube 3 through passage The photo-tube controls article discharging means comprising a first stage triode amplifier l0 and a second stage triode amplifier ll. These triodes arepreferably of the type having indirectly heated cathodes l2 and [3. The amplifiers control an ionizing or power tube 14 preferably of the grid controlled gaseous discharge type, the functioning of the power tube energizing the solenoid Hi to attract the armature l5, thus ejecting undesirable articles from the conveyor by striking them with a properly placed tappet l6l attached to the armature.

Direct current potentials are provided (by means not shown) to give 250 volts, ground potential, +20 volts and volts and are connected respectively to terminals I1, l9, 2| and I8. A voltage divider circuit is provided between the 250 volt terminal and ground, comprising a 2200 ohm resistor 20, a 40 ohm resistor 22, a 1200 ohm resistor 23, a 200 ohm potentiometer 24 and a 2500 ohm resistor 25. The plate of photo-tube 8 is connected to terminal 11, and the anode of photo-tube 8 connects directly to grid of amplifier tube l0 and through 40 megohm resistor 26 to the positive terminal of resistor 20. The cathode of amplifier I0 is connected to the positive end of resistor 22, thus biasing the grid of amplifier tube In with potential drop across resistor 22. The plate of tube 10 connects to the positive terminal of resistor 23 through a composite resistor consisting of 20,000 ohm potentiometer 21 (the control dial), 1000 ohm resistor 28 and six 1500 ohm resistors 29 connected in series to consecutive points of a gang switch (range selector) whose movable contact 30 may thus be connected to any terminal.

In Fig. 2, a normalizing space 60 is provided in the rim of conveyor l by the omission of an object or article seat. Spaces 60 may be provided alternately of the article seats 2 or may be more widely distributed, say, one space 60 following a series of fifteen or twenty article seats.

A rotary switch 55 of insulating material is provided with a small conducting lug or contact 59; Brushes 56 bearing on this rotary switch make contact with the conducting lug 59 during part of the time when space 60 is traversing the inspecting zone 4 and serve to connect the grid aromas of amplifier tube ii'wlth contact 30. T e grid of amplifier i i is permanently connected through condenser 31 to movable contact 32 of potentiometer 2?. The cathode of amplifier it connected to movable contact 323 of potentiometer 24. The plate of amplifier i i is connected to the grid of gaseous discharge tube M and through 50,000 ohm resistor 23 to +20 volt terminal iii. The gaseous discharge tube cathode is centrally con nected to ground and its plate connects to brush 35 hearing on a commutator 36.

The filaments of tubes iii, ii and it erably heated by separate windings of former (not shown).

commutator 36 is so geared to drum i that one revolution of the commutator encompasses the passage of two objects. On this commutator there also bear brushes 3'! and 38 connected together with a 6000 ohm resistor 39. Brush also connects through-e00 ohm solenoid iii to positive voltage supply it. The insulating and com ducting segments provided on commutator 36 are so placed and proportioned that as an object enters the inspecting zone brushes and iii are are prefa transconnected together. Plate potential is applied to gaseous discharge tube M so that it may discharge, but solenoid i6 will not drive the armature because of the current limitation by 6000 ohm resistor 39. This condition obtains for approximately one-third of the time interval between consecutive object seats and is hereinafter referred to as the scanning period. At the termination of this period, commutator 36 connects brush 35 directly with brush 38, thus shortcircuiting resistor 39 and permitting the solenoid I6 to actuate armature l 5 and its attached tappet if tube 14 has lighted during the scanning period. This commutator condition obtains for the second one-third cycle hereinafter referred to as the stroke period. At the termination of the stroke period, brush 35 is disconnected from both brush 3'! and brush 38, which condition obtains for the remaining one-third cycle hereinafter referred to as the tappet return period. The assorting cycle is made up of the scanning, stroke, and tappet return periods and constitutes the interval between the time when the scanning of one object starts and the time when the scanning of the next object starts.

To best understand the operation of this circuit, it must be considered as performing alternately two essentially independent functions: (a) normalizing or setting, which may be defined as a specific, automatic process whereby the circuit is given a setting appropriate to certain conditions such as lamp brightness or photo-tube power, and (b) appraising or the actual sorting.

The machine assumes function (a) during the brief interval in which lug 59 of rotary switch 55 is in contact with brushes 56. Condenser 3| then ceases functioning asap electrostatic inductor because of the relatively short circuit between its terminals. The grid of amplifier II is resistance-coupled to amplifier 10 through contact 30 of the range selector switch. The plate current of amplifier l0 causes a certain total potential drop across its composite 30,000 ohm plate resistor 21, 28, 29, of whichthe grid of amplifier Il may receive 5%, 10%, 15%, 20%, 25% or 30%, depending upon the setting of the range selector. In the drawing, the pointer 30 is shown in the position which applies 10% of this potential as an automatic bias to amplifier H. An additional manually controlled bias is applied by moving contact 33 of potentiometer 24 until with lamps 1 extinguished tube l4 barely lights, its grid having been brought to the critical ignition point by gradually decreasing the plate current traversing resistor 34. I term this bias the critical bias, 1. e., the value of grid bias of the ionizing tube, which is barely sufilcient to withhold ionization. -If now the lamps I be lighted and tube ll extinguished as by opening its plate circuit, the grid of tube 14 will be found to acquire an additional bias essentially proportional to the effective brightness of the lamps. This bias I term a retaining bias, 1. e., any bias of the ionizing tubes grid in excess of the critical bias. The term efiective brightness of-the lamps is used to signify the brightness of lamps I as measured by its effect on whatever photo-tube is used, and after traversing any color filter which may be present. It will be understood that the establishment of the excess or retaining bias on the grid of tube ll takes place every time lug. 59 makes contact. -This may be a single lug as shown; eifecting'a setting once per revolution of the drum, or there may be several lugs with cor- 'responding'blank normalizing spaces 60 on rim I.

When lug 59 leaves brushes 56, the machine is set and the'assorting, or function (b), takes place in the following manner. The grid of amplifier II is left floating and is connected only to condenser 3i. Impulses originating at the phototube and amplified by tube l0 pass through the composite plate resistor. The position of the dial determines what fraction of the impulses is to proceed to amplifier II. The 20,000 ohm potentiometer 21 covers a range of from one-third to three-thirds of this impulse. For example, let

us assume that the slide 32 divides the entire plate resistor in half. The ejector mechanism will then function on any decrease in field brightness greater than 20% as may be seen from the following. A 100% decrease in lamp brightness will operate the ejector when 10% of the first amplifier plate voltage change is applied to the grid of the second amplifier, when normalizing. When assorting, five times 10% of this voltage change is applied so that only one-fifth of the change represented by total extinction is sufllcient. The dial may be calibrated 20% at this point and, this being simply the ratio of two resistors, the machine will maintain this setting indefinitely.

I have described the preferred embodiment of my invention, which has the advantages of giving full accuracy, compensationfor lamp illumination, avoiding historical error, and permitting close spacing of articles to allow high speed operation. A modified form of my invention is illustrated in Fig. 3 and is similar to the form illustrated in Fig- 1, with the exception that retary switch and its attendant brushes 56 are omitted, the wires leading thereto being permanently connected together. -A suitable grid resistor Si is interposed between the grid of tube II and contact 30. In other respects, the circuit is identical with that of Fig. 1. In the embodiment just described, it is necessary to provide standards, of brightness between each pair of be modified so that the brushes 58 connect the grid of tube l I not to contactor 30, but to the positive end of resistor 23. By such a circuit, full accuracy is obtained, historical error is avoided, and close spacing of articles on the conveyor with resulting full speed operation is made possible. However, obviously the compensation for variations in lamp illumination is lost.

Since the grid side of coupling condenser 3| has, when assorting, no connection whatever to a fixed potential point, this unit does not act as a capacity," introduces no time delay, and requires no appreciable charging current. Potential changes are transmitted essentially without alteration regardless of frequency, steepness of wave front, or duration. This gives exceedingly high precision and uniformity to the series of appraisals and completely overcomes historical As the stream of articles passesthe scanning zone, certain articles will register sufficiently to operate the ejector mechanism earlier or later in the cycle than others. If the trip were permitted to occur at a moment of suflicient darkness, the ejector would strike at times too high or too low to certainly dislodge an article. A sufilciently broad ejector blade would be reliable in removing rejected articles, but with the close spacing which the present invention makes possible, such a broad ejector would frequently strike two objects at once. Furthermore, 'such an ejector would extend either into the inspecting zone or so close to it that its shadow might interfere with the viewing mechanism.

In the present invention, the sustaining resistor I 39 permits the lighting of gaseous discharge tube I4 at any instant during a scanning period, but delays the armature stroke until theobject seat is carefully and properly located for meeting the thrust of a narrow, low-placed tappet blade. Once lighted, the tube l4 retainsa state of ionization so that an impulse sufficient to light the tube ll cannot be recalled by a subsequent brightening of the field.

By the structure just described, I have provided what may be termed an isochronous ejector mechanism, viz., an ejector and associated apparatus disposed to start an actual ejecting stroke at an invariable time in the'assorting cycle if a sufficient impulse was received at any previous portion of that cycle.

It will be apparent to those skilled in the art that I provide an assorting system which is independent of annoying variations which heretofore have affected the efficiency of photo-electric assorting devices. I have solved a vexing problem of long standing inasmuch as my system is entirely independent of constant illumination at the inspecting zone, which was a necessary and almost unattainable characteristic of assorting machines prior to my invention. My device is especially well adapted tohigh speed and is very accurate in sorting small articles such as beans and the like.

Having thus described my invention, what I claim as new and desire to secure by Letters Patent is:

1. In a photoelectric assorting apparatus, the combination of an illuminated viewing field, a photo-tube optically exposed to said field, a conveyor for introducing into said field alternately objects to be assorted and portions of a reference background, an ejector capable of ejecting certain objects from said conveyor when actuated, and an electrical amplifying system receiving impulses from said photo-tube, capable of actuating said ejector, and containing a first amplifier tube and second amplifier tube, electro-static induction means between said tubes the plate of said first amplifier causing voltage variations in the control-grid of said second amplifier by electrostatic induction while objects are being viewed, the grid of said second amplifier being essentially floating during such times, said grid being con nected through a highly conductive path to a suitable established biasing potential while said photo-tube views the reference background.

2. In a photoelectric assorting apparatus, the combination of an illuminated viewing field, a photo-tube optically exposed to said field, a conveyor for introducing into said field alternately objects to be assorted and portions of a reference background, an ejector capable of ejecting certain objects when actuated, and an electrical amplifying system receiving impulses from'said photo-tube, capable of actuating said ejector, and containing a first amplifier tube and a second amplifier tube, electro-static induction means between said tubes, the plate of said first amplifier causing voltage variations in the controlgrid of said second amplifier by electrostatic induction while objects are being viewed, the grid of said second amplifier being essentially floating during such times, said grid being connected through a highly conductive path to a biasing potential varied by direct current amplification of the photo-tube current while said photo-tube views the reference background.

3, In a photoelectric assorting apparatus, the combination of a viewing field, a photo-tube optically exposed to said field, a conveyor for introducing into said field periodically objects to be assorted and portions of a reference background, an ejector capable of ejecting certain objects when actuated, and an electrical amplifying system receiving impulses from said phototube, capable of actuating said ejector, and containing a first amplifier tube and a second amplifier tube, electro-static induction means between said tubes, the plate of said first amplifier causing voltage variations in the control-grid of said second amplifier by electrostatic induction while objects are being viewed, the grid of said second amplifier being essentially floating during such times, said grid being connected through a highly conductive path to a biasing potential varied by direct current amplification of the photo-tube current while said photo-tube views the reference background.

4. In a photoelectric assorting apparatus, the combination of a viewing field, a photo-tube optically exposed to said field, a conveyor for'introducing into said field periodically objects to be assorted and portions of a reference background, an ejector capable of ejecting certain objects when actuated, and an electrical amplifying system receiving impulses from said phototube, capable of actuating said ejector, and containing a first amplifier tube and a second amplir' fier tube, electro-static induction means between said tubes, the plate of said first amplifier causing voltage variations in the control-grid of said second amplifier by electrostatic induction while objects are being viewed, said grid being connected through a highly conductive path to a biasing potential while said photo-tube views the reference background.

,5. In a photoelectric assorting apparatus, the combination of a continuous conveyor having spaced article seats and portions of a reference background thereon, photoelectric means arranged to view articles in said seats and said portions, means to illuminate the conveyor and. articles, means controlled by said photoelectric means for ejecting articles from said conveyor including a grid-controlled gaseous discharge tube so connected that its plate current causes and determines the ejecting operation, means acting to periodically normalize the apparatus when said photoelectric means views said reference portions, and means further timing and controlling said ejecting means comprising a commutator driven in timed relation with said conveyor, and brushes bearing against said commutator and disposed to apply plate voltage to the grid-controlled gaseous discharge tube during scanning periods through a circuit incapable of operating said ejector and to apply plate voltage to the grid-controlled gaseous discharge tube during ejecting periods, whereby ejection of an article occurs at an invariable point in each as sorting cycle.

6. In a photoelectric assorting apparatus, the combination of a continuous conveyor having spaced article seats and portions of a reference background thereon, photoelectric means arranged to view articles in said seats and said portions, means controlled by said photoelectric means for ejecting articles from said conveyor including a grid-controlled gaseous discharge tube so connected that its plate current causes the ejecting operation, means acting to periodically normalize the apparatus, and means further timing and controlling said ejecting means comprising a commutator driven in timed rela tion with said conveyor, and brushes bearing against said commutator and disposed to apply plate voltage to the grid controlled gaseous discharge tube during scanning periods through a circuit incapable of operating said ejector and to apply plate voltage to the grid-controlled gaseous discharge tube during ejecting periods through a circuit capable of operating said ejector, whereby ejection of an article occurs at an invariable point in each assorting cycle.

7. In a photoelectric assorting apparatus, the combination of a continuous conveyor having spaced article seats and portions of a reference background thereon, photoelectric means arranged to view articles in said seats and said por tions, means controlled by said photoelectric means for ejecting articles from said conveyor including a grid-controlled gaseous discharge tube so connected that its plate current causes the ejecting operation, and means further timing and controlling said ejecting means comprising a commutator driven in timed relation with said conveyor, and brushes bearing against said commutator and disposed to apply plate voltage to the grid-controlled gaseous discharge tube during scanning periods through a circuit incapable of operating said ejector and to apply plate voltage to the grid-controlled gaseous discharge tube during ejecting periods through a circuit capable of operating said ejector, whereby ejection of an article occurs at an invariable point in each assorting cycle.

8. In a photoelectric assorting'apparatus, the combination of a continuous conveyor having spaced article seats and portions of a reference background thereon, photoelectric means arranged to view articles in said seats and said portions, solenoid means controlled by said photoelectric means for ejecting articles from said conveyor including a grid-controlled gaseous discharge tube so connected that its plate current causes the ejecting operation, and means further timing and controlling said ejecting means com prising a commutator driven in timed relation with said conveyor, and brushes bearing against said commutator and disposed to connect said ejector solenoid to the plate circuit of said gridcontrolled gaseous discharge tube through a resistor during scanning periods and disposed to short-circuit said resistor at a fixed point in the asserting cycle.

9. In a photoelectric assortlng apparatus, the combination of a continuous conveyor having spaced article seats and portions of a reference background thereon, a photoelectric cell arranged to view articles in said seats and said reference background, means controlled by said photoelectric cell for ejecting off-color articles from said conveyor, means acting to periodically normalize the apparatus with respect to the light reflected from said spaced reference portions to said cell and comprising first and second amplifiers in the control circuit of said ejecting means, a condenser in the plate-grid circuit of said amplifiers, means to periodically establish a grid biasing circuit around said condenser when said photoelectric cell views a reference portion, and means to vary the extent of the bias so applied to said second amplifier, and means further timing and controlling said ejecting means, whereby ejection occurs at an invariable point in the as sorting cycle.

10. In a photoelectric assortlng apparatus, the combination of a continuous conveyor having spaced article seats and portions of a reference background thereon, a photoelectric cell arranged to view articles in said seats and said reference background, means controlled by said photoelectric cell for ejecting off-color articles from said conveyor, means acting to periodically normalize the apparatus comprising first and second amplifiers in the control circuit of said ejecting means, a condenser in the plate-grid circuit of said amplifiers, means to periodically establish a grid biasing circuit around said condenser when said photoelectric cell views a reference portion, and means to vary the extent of the bias so applied to said second amplifier, and means further timing and controlling said ejecting means, whereby ejection occurs at an invariable point in the assortlng cycle.

11. In a photoelectric assortlng apparatus, the combination of a conveyor having spaced article seats and portions of a reference background thereon, a photoelectric cell arranged to view articles in said seats and said reference background, means controlled by said photoelectric cell for ejecting off-color articles from said conveyor, means acting to normalize the apparatus comprising first and second amplifiers in the control circuit of said ejecting means, a condenser in the plate-grid circuit of said amplifiers, means to periodically establish a grid biasing circuit around said condenser, and means to vary the extent of the bias so applied to said second amplifier, and means further timing and controlling said ejecting means, whereby ejection occurs at an invariable point in the assortlng cycle.

12. In a photoelectric assortlng apparatus, the combination of a conveyor having spaced article seats and portions of a reference background thereon, a photoelectric cell arranged to view articles in said seats and said reference background, means controlled by said photoelectric cell for ejecting off-color articles from said conveyor, means acting to normalize the apparatus comprising an amplifier in the control circuit of said ejecting means, a condenser, means to periodically establish a grid biasing circuit around said condenser, and means to vary the extent of the bias so applied to said amplifier, and means further timing and controlling said ejecting means, whereby ejection occurs at an invariable point in the assortlng cycle.

13. In a photoelectric asserting apparatus, the combination of a conveyor having spaced article seats and portions of a reference background thereon, a photoelectric cell arranged-to view articles in said seats and said reference background, means controlled by said photoelectric cell for ejecting off-color articles from said conveyor, and means acting to normalize the apparatus comprising an amplifier in the control circuit of said ejecting means, a condenser, means to periodically establish a grid biasing circuit around said condenser, and means to vary the extent of the bias so applied to said amplifier.

14. In a photoelectric assortlng apparatus, the combination of a conveyor for transporting articles to be assorted, a photoelectric cell arranged to view articles on said conveyor, means controlled by said photoelectric cell for ejecting offcolor articles from said conveyor, means for actuating said ejection means including an ionizing power tube and electrostatic induction means between said photoelectric cell and said power tube, means to periodically establish a normalizing grid biasing circuit around said electro-static inductor means, and means to vary the extent of the bias so applied, whereby to adjust the bias of said tube proportional to the illumination of the viewing field of the photoelectric cell.

15. In a photoelectric assorting apparatus, the combination of a viewing field, lamps for illuminating said field, a light-sensitive electrical unit optically exposed to said field, means for introducing into said field at certain times objects to be assorted and at certain other times a standard of reflectivity, an ejector capable of ejecting objects when actuated, an electrical amplifying system including a grid-controlled gaseous discharge tube, said system being sensitive to currents from the light-sensitive electrical unit and capable of actuating the ejector at suitable times by the plate current of the grid-controlled gaseous discharge tube, and means for periodically adjusting the retaining bias of said grid-controlled gaseous discharge tube essentially proportional to changes in the effective brightness of said lamps, said periodic adjusting means fully effecting any requisite change of the retaining bias in a single periodic adjusting operation.

16. In a photoelectric amplifier, the combination of a light-sensitive electrical unit, a viewing field therefor, an electrical amplifying system including a grid-controlled gaseous discharge tube, and means for periodically adjusting the retaining bias of said grid-controlled gaseous discharge tube essentially proportional to changes in the illumination of said viewing field, said .periodic adjusting means fully effecting any requisite change of the retaining bias in a single periodic adjusting operation.

17. In a photoelectric assortlng apparatus, the

combination of a viewing field, lamps for illuminating said field, a light-sensitive electrical unit optically exposed to said field, means for introducing into said field at certain times objects to be assorted and at certain other times a standard oi reflectivity, an ejector capable of ejecting objects when actuated, an electrical amplifying system including a grid-controlled gaseous discharge tube, said system being sensitive to ,currents from the light-sensitive electrical unit and capable of actuating the ejector at suitable times by the plate current of the grid-controlled gaseous discharge tube, and means for adjusting the retaining bias of said grid-controlled gaseous discharge tube essentially proportional to the effective brightness of said lamps, said adjusting means fully effecting any requisite adjustment in a single periodic adjusting operation.

18. In a photoelectric asserting apparatus, the combination of an illuminated viewing field, a photo-tube optically exposed to said field, a conveyor for introducing into said field alternately objects to be assorted and portions of a reference background, an ejector capable of ejecting objects when actuated, and an electrical amplifying system receiving impulses from said photo-tube capable of actuating said ejector, and containing a first amplifier tube and a second amplifier tube, electro-static induction means between said tubes, the plate of said first amplifier causing voltage variations in the control grid of said second amplifier by electrostatic induction, said grid being also connected through a highly resistive path to a biasing potential varied by direct current amplification.

19. In a photoelectric assorting apparatus, the combination of a viewing field, a reference background, a photo-tube optically exposed to said field and reference background, means for introducing into said field objects to be assorted, an

. ejector capable of ejecting certain objects from causing voltage variations in the control-grid 01' said second tube by electro-static induction while objects are being viewed, the grid of said second tube being essentially floating during such times, said grid being connected through a highly conductive path to a suitably established biasing potential while said photo-tube views the reference background.

20. In a photoelectric assorting apparatus, the combination of a viewing field, a reference background, a photo-tube optically exposed to said field and reference background, means for introducing into said field objects to be assorted, an ejector capable of ejecting certain objects from said means when actuated, and an electrical amplifying system receiving impulses from said photo-tube, capable of actuating said ejector, and containing a first amplifier tube, a second amplifier tube, and electro-static induction means between said tubes, the plate of said first tube causing voltage variations in the control-grid of said second tube by electro-static induction while ob jects are being viewed, the grid of said second tube being essentially floating during such times, said grid being connected through a highly conductive path to a biasing potential varied by direct current amplification of the photo-tube current while said photo-tube views the reference background.

21. In a photoelectric asserting apparatus, the combination of a conveyor having spaced article seats, a photoelectric cell arranged to view articles on said seats, means controlled by said photoelectric cell for ejecting oil-color articles from said conveyor, and means acting to normalize the apparatus, comprising an amplifier in the control circuit of said ejecting means, a condenser, means to periodically establish a grid biasing circuit around said condenser, and means to vary the extent of the bias so applied to said amplifier.

DAVID C. COX. 

